I couldn't justify it though, since I have the 14/2.5 which is not a whole lot bigger.

I also have the 14/2.5 and the 17/2.8. I ordered the 15/8 for fun. If the upcoming 17/1.8 is worth purchasing, I would replace the 14/2.5 and 17/2.8 with it. Then the 15 would make sense as a backup, attached to my GF3. One mean pocketable camera.

As a thought, is there room on the back for adding a dandelion chip?

There should be enough room for the Dandelion. It would be great to add it to the Samyang 7.5 also.